Collimating

From Scope F70076

There are several methods of collimating, it can be done using a laser, or by the diffraction rings around a star when the telescope is slightly out of focus. I though satisfy with only the easiest collimation, as I think that taking care of such telescope shouldn't take too much time.

Collimating is aligning the primary and secondary mirror. The mechanism which enables collimation is really the most simple, I would even say primitive, and this mechanism is the same both for the primary and secondary mirror. There is a plate with three screws on it, which are called the collimation screws. These screws go against some support, which is the bottom of the telescope tube in case of the primary mirror. This simply means that by adjusting these screws by a few turns, the tilt of the mirror can be changed. And there are the other screws called locking screws, which simply fasten the plate to the support. There are three locking screws for the primary mirror and one locking screw for the secondary mirror. Naturally, for adjusting the tilt of a mirror the locking screws must be loosened. For the primary mirror you should loosen all screws first only slightly, and then all almost by the same amount, as doing that unevenly would cause one screw to become too tight. The collimation screws are in a triangle, which means that for tilting towards one screw, the two opposite screws must be evenly screwed in.

Collimate the secondary mirror only when the telescobe tube is in the horizontal position, to avoid anything from falling into the tube, which may damage the primary mirror. The first thing which you likely should do for collimating, is to look through the focuser (in the lowest position and without any eyepiece in) whether you can see the whole primary mirror. If you cannot, you should either adjust the tube axis position of the secondary mirror, or the spider (with which the secondary mirror is fastened to the tube) so that the mirror would be closer to the focuser. For that you may need to remove the spider, and adjust the length of its vanes, all are also screwed into the center, so they can be adjusted that way too. It seems to be a problem with these telescopes that the secondary mirror is only then close enough to the focuser, when the center of the spider is not exactly in the center, or the secondary mirror seems to be a bit too small. Then collimate the secondary mirror so that you can see the whole primary mirror.

Then collimate the primary mirror so that the spider is in exactly the same shape as you see it when you look into the tube (unfortunately we cannot say in the center). There is also a thing called collimation cap, which for example can be made of a 35mm photo canister by making a small hole in its center, putting it into the focuser and looking through that hole. The only purpose of the collimation cap is to ensure that your eye is in the center. But we can satisfy with less precision, and therefore not to use even this additional tool.